The present invention relates to a form of screw head for a screw fastener, and to a tool to drive such screw. In particular, the invention relates to a method of manufacture of a punch for cold forming such screw head.
GB1150382 appears to be the first patent to recognise the benefits of a multi-tiered screw head design that has stepped tiers between a deep central recess and a wider top recess, the recess having sides parallel a central axis of the screw. Such recesses are effective at reducing xe2x80x9ccam outxe2x80x9d. Furthermore, the recess can be deeper in the head of the screw without weakening the connection between the shaft of the screw and its head. This is because, at its deepest, where the countersink is narrowest or the transition between head and screw shank occurs, the recess is smallest.
GB2285940 elaborated on this principle and pointed out that the same screwdriver could be employed to drive large screws (having several tiers of recesses in their heads) as well as small screws having fewer or even just one tier of recess in their heads. Thus the proliferation of tools can be avoided as just one driver will fit, completely appropriately, a number of differently sized screws.
GB2329947 discloses a similar arrangement and discusses the method of manufacture of screws incorporating multi-tiered heads.
Such manufacture employs xe2x80x9ccold headingxe2x80x9d. A punch, being the mirror of the recess desired, is driven into the unformed head of the screw. Momentarily, the metal is fluidised and flows around the shape of the punch.
Where machine screws are being constructed, it is common to provide them with a single tiered polygon recess. Allen, Roberts and Torx (registered trade marks) are all variations of this. In common, however, they have a deep recess which is constructed to tolerances of the order of xc2x10.25 mm. This degree of precision is adequate because the area of torque transmission (given the depth of the recess) is always sufficient to avoid rounding-out).
However, such a deep recess can neither be formed without substantial draw (to permit release of the punch after forming of the recess), nor can it be formed in a single strike. The draw does not result in significant cam-out problem because of the depth of the recess. But the multiple strike using progressively larger punches does result in greatly increased costs. On top of that the head must be relatively large to provide the necessary depth. As a result of this, the three types of polygonal recess mentioned above are not employed, at least not to a known significant extent, in countersunk wood screws.
In this market, cost is vitally important. Only various cross-heads or slotted-heads are common in counter-sunk wood screws. However, this is not only because of the cost associated with the other polygon recesses. Such constant cross section recesses cannot be deep in a coned (counter-sunk) wood-screw head. Moreover, heavy torque transmission capabilities provided by polygon-recessed screws are not normally necessary.
Despite the apparent advantages of the multi-tiered screw-head design, however, they have not yet succeeded in penetrating the market. The reason for this is simply that hitherto it has not been possible to put the idea into practical, that is to say, commercial, effect.
In order to construct multi-tier-recess, countersunk wood-screws at a cost that is competitive with existing cross-head screws, such as Posi-Driv (registered trade mark), certain criteria must be met:
1) the screw heads must be capable of being formed with a single strike of a punch. Otherwise process times and punch costs (for example, if two different punches are required) render the operation prohibitively expensive;
2) punches must have a strike capacity in excess of 100,000, or thereabouts, for the same reason; and
3) punches must be inexpensive to manufacture.
The present invention resides in two observations, based on problems experienced. One of these problems is that, with parallel sided recesses; punches tend to break because of the elastic grabbing of the tip of the punch (which tip forms the deepest, smallest recess) as the punch is withdrawn. This problem can be cured partly by making the recesses very shallow.
Secondly, by increasing the tolerance (that is, by making the screws to more precise dimensions), far from this increasing the cost as would be expected, in fact, it makes the achievement of the above criteria, or at least the first two of them, possible.
Accordingly, it is an object of a first aspect of, the present invention to provide a screw of the multi-tiered recess type which is capable of manufacture in a cost effective manner.
In accordance with this first aspect, there is provided a screw having in its head at least two noncircular super-imposed recesses, a smaller one thereof in the base of a larger one thereof, wherein: the depth of each recess is less than 1.35 mm, preferably less than 1.2 mm; there is a less than 1xc2x0 of draw in each recess; and where the tolerance in the cross-sectional dimensions of at least one recess is better than xc2x10.025 mm.
These parameters are not independent of one another but are a precisely calculated combination:
1) with such a shallow depth of recess a draw of less than 10 is possible without grabbing of the punch by elastic recovery of the material. In fact, substantially no draw is preferred.
2) with such a shallow depth of recess the recess can be formed by a single strike of the punch which renders the screw relatively inexpensive to manufacture. Moreover, the punch is likely to last for in excess of 100,000 strikes, which likewise leads to cost reduction. Further, although not as a result of this aspect of the invention, the punch can be made relatively inexpensively by a method described further below and this also impacts the cost of manufacture of the screw.
3) with such accurate tolerance and low draw the tendency of the driver to cam-out or round-out of the recess is reduced, even with such a shallow recess.
4) with such a shallow recess, and the other cost factors mentioned above, the head can economically be formed on counter-sunk wood screws so that the benefits first envisaged by GB1150382, and more recently in GB2285940, can at least be realised.
Incidentally, a parameter not mentioned above but of significant importance is the hardness of the material employed. The present parameters are designed for the material from which wood screws are commonly made. However, the same principles would apply to softer or harder materials, because the depth of recess that could be formed with a, single strike is of course larger with softer material (or less, with harder material). But the corollary of this is that a deeper recess is required with softer material, whereas a shallower recess will give equal security where the material is harder. Therefore the depth of 1.2 mm specified herein has to be read in the context of the material being employed, and will be less with harder than the material common in wood screws, or more in softer materials.
In any event, however, unless the punch is capable of being made to the tolerance specified and, moreover, at a cost that meets the third criterion mentioned above, the first aspect of the present invention is to no avail.
Accordingly, it is an object of a second aspect of the present invention to provide a method of manufacture of a punch which satisfies the foregoing objective.
In accordance with this second aspect, there is provided a method of manufacture of a cold-forming punch for screw heads intended to have a multi-tiered recess, each recess being a straight-sided polygon whose sides are substantially parallel the axis of the recess, said method comprising the step of adapting a computer numerical controlled machining centre having a bar feed to a rotary chuck by providing a profilating head as one of the tools on a turret mounted adjacent the chuck and capable of both axial and radial movement with respect to the axis of the chuck, as well as rotation about its own axis, said profilating head comprising at least two cutting elements arranged on the head, the cutting elements being adjustable with respect to one another: axially, with respect to the head axis; radially, with respect to said axis; and angularly with respect to said axis, whereby the cutting elements can be arranged to rotate around the same circle and with the requisite angular separation that at least some of the sides of a polygon are formed on the end of the bar when the cutter is rotated in the same direction, with respect to the bar, as the bar is rotated in the chuck, and the turret is moved axially and/or radially with respect to the chuck to bring the cutter into contact with an end face of the bar.
Preferably, there are as many cutting elements as there are sides to be cut of the polygon.
Preferably there are at least two sets of axially and radially offset cutting elements on the cutter adapted to cut different tiers of the punch at the same time. Said cutting elements may be of the same number in each set and may be angularly offset with respect to one another.
Preferably said turret has tools to slot the punch and to pare a formed punch from the feedstock bar, whereby the entire cutting steps in the formation of the punch may be completed on a single machine.
Preferably, there are at least two profilating heads on the turret each to cut different tiers of the punch. Indeed, one head may have a different number of cutting elements to the other so that a different polygon is formed on one tier compared to that formed by the other head.
After one tier is cut, the profilating head may be indexed angularly with respect to the chuck so that the second tier is offset angularly with respect to the first.
Preferably, at least each cutting element after a first one thereof is fixed in a cartridge that is angularly adjustably retained in a radial slot in an end face of the head, the cutting element being radially and axially adjustably secured in the cartridge.
Preferably the head is provided with radial datum faces on said slots provided at the requisite angle with respect to the first cutting element, shims being employed between the cartridge and said datum face to adjust the angular position of subsequently inserted cutting elements with respect to the first cutting element to be in place.
Preferably the cutting element is secured to the base of a recess in the cartridge by a fixing screw arranged parallel the axis of rotation of the head and passing through an oversize aperture through the cutting element.
Preferably, a grub screw is threaded in the cutting element radially offset from said fixing screw and abutting the base of said recess to provide axial adjustment of subsequent cutting elements with respect to the first.
Preferably, a wedge is disposed between a radially inner face of the cutting element and an inner end of the recess in the cartridge, an adjusting screw through the wedge and received in the base of the recess pulling the wedge into engagement between said inner face of the cutting element and said inner end of the recess to provide radial adjustment of subsequent cutting elements with respect to the first.
Preferably the first cutting element is the same as said subsequent cutting elements.
Preferably said angular adjustment is performed first, with said axial and radial adjustments being performed second in an iterative process until all the cutting elements rotate about the same circle as said head rotates in the turret.
By virtue of the three degrees of freedom of each cutting element it takes considerable skill and patience to adjust the cutting elements to reach the desired precision of position. The desired precision is in the order of xc2x12.5 microns in each direction. However, once the system is set up, it is simply a matter of turning the CNC machine on and letting it run. Once set-up, operation and production of punches can be automatic.
Profilating is a known art. However employing it to the precision anticipated by the present invention has not been contemplated hitherto. Precision of the order called for by the present invention would conventionally have been achieved by, for example, milling and grinding operations. However, using such processes immediately introduces expense and complication in relation to bodies of the shape required by punches according to the present invention. Moreover, to provide a fillet between tiers which serves to support each tier and reduce the tendency for tips of punches to snap-off, is not straightforward with milling and grinding operations. Thus the second aspect of the present invention enables the first aspect to be put into effect in a cost-effective manner.
In a third aspect, the present invention provides a screw having in its head at least two non-circular, super-imposed recesses, a smaller one thereof in the base of a larger one thereof, wherein there is less than 10 of draw in each recess and wherein the edge between the recesses is chamfered. The chamfer is preferably a radius of not less than 0.05 mm, preferably about 0.1 mm.
This chamfer does not greatly reduce the area of the faces of the recess that transmit torque between a correspondingly shaped tool inserted into the recesses. But it does have a marked effect on the lifetime of a punch that forms the recesses by cold-heading. As a consequence, the recesses can be deeper than would otherwise be possible, so regaining any lost area by virtue of the chamfer.